CHEMICAL AND ISOTOPIC LABELING (CL-,(18)0 ) OF AN AUTUMNAL FLOOD FOR A SMALL BURNT MEDITERRANEAN BASIN

After the forest fire that swept across the Massif des Maures (Souther n France) in August 1990, a drastic change in the hydrological cycle w as observed on the small burnt catchment of Rimbaud (Real Collobrier E RB). Along with the usual hydrological and hydrochemical measurements, stable isotopes were measured for rainfall and runoff. Vegetation dis appearance and, consequently, the influence on throughfall encouraged a comparison of oxygen IX content with chloride content. During the fl ood of 9 December 1990, these two elements showed a very similar behav iour, and it was interesting to notice that the hydrograph separations obtained by isotopic methods were comparable. When the usual factors of chloride and oxygen 18 evolution were examined, 45% usual factors o f chloride and oxygen 18 evolution were examined, 45% of runoff came f rom ''ancient'' water and, in this case, the unsaturated area did not play a significant role. Moreover, as regards the behaviour of the wat er temperature indicating the end of runoff, one could estimate the ne w water amount infiltrating to the water table at less than 30%.